That's just how our society is. We give attention and listen to those that should be ignored, and folks like Stephen Hawking are put in the background, with their "wild theories and imagination".
Only posthumously does the media and people seem to care. Le sigh.

An artist's work always gets more valuable after their death, both in the literal, financial sense, and emotionally.

I guess it's because, after a person dies, they can't make any more mistakes: no more silly scientific theories, no more less-than-stellar new songs, no more social gaffes. They are done -- they can fail no longer -- so society can now pick-and-choose the best parts and remember only those, without the person behind them being around to remind us that they're human and flawed.

To be fair, I never thought he didn't know what he was talking about.

Death has such an ironic effect on everyone.

Also I remember there were people who would also claim that they're sick of Robin Williams and I'm sure there were those who would also make jokes and insults towards him. Then when he dies everyone is upset by it. Same story for Satoru Iwata (former president of Nintendo) I still recall everyone would constantly bitch about him and blame him as the reason why Nintendo was going downhill and even make fun of him and insult him. Yet he dies and everyone suddenly has respect for the man and starts acknowledging his accomplishments and contributions to Nintendo and the many games he worked on.

It's a sad look at how many people are never truly appreciated and loved until they are gone.

For Micheal Jackson, I can't say it's wrong. But for Stephen Hawking, I don't know, it always seemed to me everyone were respecting the guy as a major scientist and his work was more than one time proven valuable. One can say "suddendly everyone find his work interresting when they weren't", but I don't think I ever saw someone say "yeah, what a know-nothing loser" ever.